Research reveals new facts about nematode pests

The highlights from three Hortgro-funded projects. By Jorisna Bonthuys.

Three projects funded by Hortgro are generating new knowledge about plant-parasitic nematodes. The studies focus on whether different cover crops host any of three lesion-nematode species, whether new apple rootstocks are resistant or tolerant to lesion nematodes, and whether certain soils suppress ring nematodes. Fresh Quarterly spoke to project leader Dr Rinus Knoetze, senior researcher at the ARC Infruitec-Nietvoorbij, to learn more.

Cover crops as potential nematode hosts

Root-lesion nematodes — Pratylenchus species — are a major apple-orchard pest in South Africa. These nematodes are migratory endoparasites that feed inside roots, tunnelling back into the soil and to new roots at will. Infected apple trees, especially younger trees, display poor growth, and yields of older orchards may decline over time. Root-lesion nematodes have also been implicated in apple replant disease.

Knoetze recently completed a study on the host status of certain cover crops for root-lesion nematodes. He modified a laboratory method that enabled him to mass culture the three lesion-nematode species most prevalent in local apple orchards. Knoetze cut sterilised carrots into coin-sized discs before inoculating them with lesion nematodes. The nematodes were left to multiply for two months, and then extracted in large numbers.

After propagating cover crops, Knoetze infected the plants by burying nematode-infested carrot discs close to them. He also planted cover crops next to apple rootstocks infected with lesion nematodes. Later, he extracted nematodes from plant roots, and determined how many nematodes were present per gram of root sample analysed.

The results showed that some crops — like nasturtiums, and Indian buckwheat — are moderate to good hosts for the root-lesion nematodes that occur in apple orchards. Other cover crops — like pink serradella, rye, and subterranean clover — are good hosts for Pratylenchus penetrans only.

“If these crops are used as cover crops in apple orchards, it could cause an increase in nematode numbers if planted at high density,” cautions Knoetze.

Crops like perennial ryegrass, Saia oats, tillage radish, mustard, canola, plantain, and marigolds do not support significant reproduction of the lesion-nematode species tested. As such, they can be used in South African apple orchards without contributing to the lesion-nematode problem.

Knoetze found that reproduction of the most common lesion-nematode species, P. hippeastri, is minimal on most cover crops in winter. Still, it can increase on certain crops — pink serradella, rye, triticale, subterranean clover, and medicks — during spring and summer if these crops persist in the orchard.

The study has contributed to a database of cover-crop characteristics which enables growers to select the best options for their specific circumstances. “If the species of lesion nematode in an orchard is known, producers can now make a more informed decision about the choice of cover crops,” says Knoetze.

“The project has increased our understanding of these plant-parasitic nematodes in orchard floor management significantly,” says Matthew Addison, crop-protection programme manager at Hortgro Science.

Furthermore, this study has made another valuable contribution by fine-tuning the carrot-disc method for cultivating nematodes, which will support future research efforts. “The availability of large numbers of lesion-nematode inoculum will enable us to take on projects where such high numbers of inoculum are required,” explains Addison. “These include rootstock screening trials for nematode resistance, and the evaluation of control measures.”

Screening rootstocks for resistance to lesion nematodes

“Very few of the current commercially available apple rootstocks are known to be tolerant of or resistant to these nematodes, so this is a problem you really want to avoid,” says Knoetze. Once established in an orchard, root-lesion nematodes are difficult to control.

In an ongoing project, Knoetze is investigating the resistance or tolerance of apple rootstocks to the most common root-lesion nematode species — Pratylenchus hippeastri, P. vulnus, and P. penetrans — found in South African orchards. The aim is to evaluate new rootstocks, including the promising Geneva rootstocks.

Knoetze obtained selected rootstocks produced by tissue culture. All rootstock batches were screened to ensure freedom from lesion nematodes. The researchers then employed the same carrot-disc technique used in the study above to cultivate nematodes and inoculate plants.

One growing season after inoculation, Knoetze determined how many nematodes were present per gram of root.

Preliminary results indicate that the rootstocks he screened are susceptible to the root-lesion nematode species that occur in South African apple orchards. In addition to the laboratory study, Knoetze is also assessing the prevalence of lesion nematodes on rootstocks at industry rootstock-evaluation sites.

The project continues until March 2022.

Studying soils that suppress ring nematodes

Certain healthy soils can suppress plant-parasitic nematodes, preventing them from establishing and from causing disease. Hortgro and SATI are jointly funding a project to better understand the characteristics of naturally nematode-suppressing soils.

According to Addison, this research can help clarify how soil ecology affects nematode diversity. “The suppression mechanism may well be common to nematodes in general, in other words not a rootstock characteristic.”

The research team is focussing on soils that suppress ring nematodes — Criconemoides xenoplax. Ring nematodes are important pests of stone fruit, and among the most abundant nematode species in vineyards in the Western Cape, so suppressive soils are of interest to both stone-fruit and table-grape producers.

Nematicides are not very effective against ring nematodes, because they occur deep underground — up to one metre below the surface — where nematicides do not reach.

Evidence suggests that the biological control of nematodes occurs naturally in many areas, this control being related to the specific soil microbiome. There are certain field sites where plant-parasitic nematodes are maintained at very low population densities by one or more indigenous microorganisms.

Knoetze thinks that new biological agents could be found in these soils. “Hopefully, our investigation of natural suppressive soils will provide more techniques for inducing plant-parasitic nematode control.”

The researchers plan to assess nematode diversity in each soil sample. They are also interested in how crop management practices can help support the biological control of problem nematodes. Changes in soil composition related to these practices can increase the number of beneficial nematodes present, while decreasing plant-parasitic nematodes.

The project continues until March 2022.